1. Field of the Invention
The present invention relates to an image pickup apparatus which performs focus detection by a contrast method.
2. Description of the Related Art
Conventionally, image pickup apparatuses which have a live view function and perform auto focus detection (AF control) by a contrast method have been known. The AF control by the contrast method requires calculating contrast evaluation values while changing a focus position of a focus lens to determine a position at which a contrast evaluation value reaches a peak.
In order to increase in speed of the AF control by the contrast method, a configuration may be adopted in which a frame rate during the focus detection is switched to a high-speed frame rate to reduce a sampling cycle of a contrast evaluation value, thereby a focus lens is driven at higher speed for detection of the contrast evaluation value. In this configuration, a frame rate during the AF control is set higher than that set during a normal state, taking a processing load of a system (the image pickup system), a battery remaining time depending on consumption current, and other factors into consideration. Japanese Patent Laid-Open No. (“JP”) 2013-25107 discloses an image pickup apparatus which performs the AF control with a frame rate being switched to 120 fps by pressing a release button half way when the frame rate during live view is 60 fps and sets the switched frame rate to the original frame rate of 60 fps after completion of the AF control.
In order for an image pickup apparatus to start a shooting, it is typically required that the AF control be completed to cause a focus lens to be located at an optimum focus position and photometry processing to determine an exposure during the shooting be completed. Such photometry processing is required to be performed after an in-focus frame (a focus frame) is fixed in a multi-point AF or other mode in which a focus detection frame (an AF frame) to be used for focusing is not fixed.
However, the image pickup apparatus disclosed in JP 2013-25107 switches a frame rate from 120 fps to 60 fps after the AF control to set a normal live-view state when the release button is pressed all the way at once during the AF control. The image pickup apparatus then permits a shooting after performing photometry to determine an exposure set for the shooting. Since a frame rate switching typically requires a predetermined time, a focus state may still not be optimized from the viewpoint of a release time lag even when the AF control is performed at higher speed.
On the other hand, in a configuration designed to keep, for a predetermined time during the AF control, a state in which an original frame rate is switched to a high-speed frame rate, a battery life is shorter due to increase in consumption current and a live-view operation duration is shorter due to rise in temperature, compared with a configuration in which a frame rate is returned to a normal frame rate immediately after the AF control.
In addition, the image pickup apparatus disclosed in JP 2013-25107 is configured to start the focus detection by the contrast method after changing a frame rate from a low frame rate to a high frame rate. This makes it impossible to start the focus detection until completion of the frame rate change, which in turn requires a great deal of time for the focus detection.